It may not be from breastfeeding. I read somewhere that we use different muscles holding/carrying/rocking babies that we did just standing normally pre-baby and that can make your butt sort of dissappear. Also I think the abs may relax while the back is flexing to carry the weight & that makes things weird too
I don’t think it has anything to do with breastfeeding itself, but it may have to do with the caloric intake needed to continue breastfeeding. You have to eat 500+ calories than you normally would in a “maintenance” phase. I think once you’re in a true caloric maintenance or deficit + going to the gym, you’ll see the muscle definition returning.
I heard that breast-feeding takes the fat out of your butt and other fatty areas and puts it into your boobs. I don’t know if that’s true though.